This project comprises the development, evaluation, and utilization of a method for determining the kinetic parameters of metabolism of vitamin D3 and its metabolites. The procedure developed is based on compartment analysis after injection of a trace amount of labeled vitamin D3 or its metabolites and permits quantitation of pool sizes, rate constants, and transfer rates. This technique will be utilized in man to examine the effect(s) of induced perturbations and to define the quantitative abnormalities present in various disease states known or thought to be associated with clinical defects in vitamin D metabolism.